It is well known that antenna diversity can improve the reception quality of communications in a wireless environment and yield increased system capacity. Conventionally, selection diversity is the simplest diversity scheme which operates on the principle of selecting the antenna diversity branch which provides the strongest received signal level or the best eye-opening. However, it is known that selection diversity does not provide any useful gain in a line-of-sight (LOS) environment since the two branches are correlated.
In a recent paper by Cox and Wong, "Low-Complexity Diversity Combining Algorithm and Circuit Architectures for Co-channel Interference Cancellation and Frequency Selective Fading Mitigation", IEEE Trans. Comm. Vol, 44, no 9, pp. 1107-1116, September 1996, it is shown that two antenna optimum-combining diversity produces a signal-to-interference ratio (SIR) improvement of at least 3-dB over conventional two-antenna selection diversity in Personal Access Communication Systems (PACS). This is attractive since combining diversity can be applied to cancel co-channel interference and boost the desired signal even in an LOS environment.
Qualitatively speaking, in an LOS environment, an optimum-combining receiver adjusts the joint signal of a plurality of antennas, resulting in an adaptive joint antenna pattern or polarization which attenuates co-channel interference while amplifying the desired signal. In a multi-path environment, the antennas may be receiving signals from separate paths and this picture is not entirely applicable, but the concept is the same.
While optimum-combining diversity offers attractive performance improvement over selection diversity, it is noticed that existing antenna diversity researches concentrate on selection diversity. Such a preference is probably due to that fact that many of the so-called adaptive antenna array solutions rely on algorithms which require well characterised antenna patterns. In contrast, most mobile PCS handset antennas possess patterns which are not carefully controlled and are quite dependent on the position of the antenna with respect to the user's hand and head. Thus, if optimum-combining diversity is to be devised and implemented on mobile PCS receiver handsets, the first task would be to seek optimum-combining diversity algorithms which do not require well characterised antennas as a prerequisite.
Hitherto, system complexity together with the associated power consumption, cost and size has been a significant barrier to the wide-spread commercial implementation of diversity schemes in PCS portable handsets since most proposed diversity handset schemes require one receiver chain for each branch of diversity which means that receiver circuitry from RF to baseband has to be duplicated. This dual receiver chain design approach is contradictory to the industrial trend of circuit simplification and consumer appetite of miniaturisation and cost reduction. This limitation, unless circumvented, would continue to hinder implementation and further development of diversity schemes in mobile handsets.
In the Cox & Wong publication above, there is shown a symbolic diagram, i.e. FIG. 1, which discloses the concept of a simplified ideal diversity receiver design in which the RF signal from two antenna branches are combined after level adjustment but before further processing presumably by a single channel device for baseband processing. However, this disclosure merely shows a future receiver topology hopefully to be implemented but the underlying algorithm proposed in that publication does not actually support implementation of a diversity receiver using single channel baseband signal processing.
Furthermore, while selection diversity algorithm does not offer significant signal quality improvement in the circumstances mentioned above, it is nevertheless very fast and energy efficient. In circumstances where the signal quality received by one of the antennas is superbly high, selection diversity would be beneficial and it would be highly desirable that the simpler selection diversity can be chosen and utilised. Thus, it would be highly desirable if a diversity receiver handset can accommodate a number of modes of diversity algorithms which can be chosen according to the reception conditions. This would of course require the presupposition that the prime constraints of low-cost, low-complexity and low-weight are observed.